In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as integrated circuits (IC) and electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices, EL (electro luminescence) display devices, and the like. In an active matrix liquid crystal display device, specifically, a voltage is applied between a pixel electrode connected to a switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements.
The application range of the active matrix liquid crystal display devices is expanding, and demands for larger screen size, higher definition, and higher aperture ratio, and the like of such a liquid crystal display device are increasing. In addition, a need exits for low manufacturing cost and high yield with respect to such an active matrix liquid crystal display device as well as high reliability. As a method for increasing productivity and reducing manufacturing cost, simplification of the process can be given.
In an active matrix liquid crystal display device, thin film transistors are mainly used as switching elements. In order to simplify a manufacturing process of thin film transistors, reduction in the number of photomasks used in photolithography is effective. If one photomask is added, for example, the following steps are required: resist application, prebaking, light exposure, development, postbaking, and the like, and moreover other steps before and after the aforementioned steps, such as film formation and etching and further resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one photomask in the manufacturing process. Thus, a process is simplified or complicated depending on the number of photomasks; therefore, many technical developments are made so as to reduce the number of photomasks used in a manufacturing process.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. There has been a concern that yield is reduced due to various problems caused by usage of such a complicated technique. Moreover, there has often been no option but to sacrifice electrical characteristics of thin film transistors.
As typical means for reducing the number of photomasks in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 (Japanese Published Patent Application No. 2003-179069) is, for example, disclosed.